1. Field of the Invention
The present invention relates to the removal of metals and ashes from spent salt generated from a molten salt oxidation reactor. In particular, this method removes primarily Group VIII metals such as chromium, nickel, iron, molybdenum, and the like, from spent salt to facilitate recycling of the salt for reuse in the reactor or disposal of the salt as non-hazardous waste.
2. Description of Related Art
Molten salt oxidation (MSO) is a thermal process that is capable of destroying organic constituents of energetic materials, hazardous wastes, and mixed wastes (i.e., wastes containing both organic and radioactive materials). In this process, combustible waste and air are introduced into a bath of molten carbonate salt (typically sodium carbonate), where the organic constituents of the waste materials are oxidized to carbon dioxide, nitrogen, and water. Inorganic products resulting from the reaction of the molten salt with the halogens, sulfur, phosphorous, metals, and radionuclides introduced into the salt bath must be removed to prevent the excessive build-up of inorganic products in the sodium carbonate. The excess build-up of these products in the carbonate salt can result in a dramatic drop in the efficiency of the system and can greatly increase the amount of toxic off-gases produced.
The carbonate salt serves both as a chemical reagent and as an acid scrubber to neutralize any acidic by-products produced during the waste destruction process. As the carbonate content in the salt decreases, the efficiency of the process decreases. At a certain point, the salt is removed from the reactor and the hazardous constituents are separated from the salt.
Because many of the metals and radionuclides captured in the salt are hazardous, the spent salt removed from the reactor can create a large secondary waste stream without further treatment. Thus, there is a need for a spent salt clean-up and recovery system to segregate these materials and minimize the amount of secondary waste. Once the hazardous constituents have been isolated, they can then be encapsulated for final disposal. This invention describes a separation strategy developed for metals and ashes removal from mixed spent salt and provides a way to reduce the consumption of fresh salt and to reduce the amount of secondary waste.
The present invention is a method for removing primarily metal contaminants and ashes from the spent salt of a molten salt oxidation (MSO) reactor. Removal of these contaminants enables the secondary waste stream generated by the MSO operation to be kept at a minimum. Once the contaminants are removed, the spent salt may either be re-used in the MSO process or disposed as non-hazardous waste. If the salt still contains a high amount of carbonate (usually greater than about 20%), it will be recycled into the MSO reactor through the apparatus of the invention. If the salt contains a low amount of carbonate (less than about 20%), it no longer serves a useful purpose in the MSO process and will therefore be disposed through apparatus of the invention. Although this invention may be applied generally to metals, and more particularly to Group I, II, and VIII metals, the processes emphasized herein will be for removing xe2x80x9creducible metallic ionsxe2x80x9d from MSO spent salt that are capable of being reduced to more insoluble metallic ionic forms of such ions.
To begin removal of the contaminants, the spent salt is cooled to ambient temperature, removed from the reactor, ground up, analyzed, and dissolved in water to form a primary salt solution. Most of the mineral residues in the spent salt have low solubilities in water, depending upon solution pH values, and are precipitated from the primary salt solution as metal oxides, metal hydroxides, and ash, during the dissolution step. The pH of the primary salt solution is normally highly alkaline, usually due to the concentration of carbonate present. Optionally, an alkali hydroxide such as sodium hydroxide and/or a sulfiding agent such as NaHS can added to the primary salt solution, causing the metals (e.g., metallic ions) present in the primary salt solution to form additional insoluble precipitates. The primary salt solution containing precipitate is filtered to yield (1) a filtrate, i.e., a secondary salt solution, and (2) a contaminated filter cake, i.e., a waste solid containing a majority of the metal contaminants of the spent salt, i.e., a xe2x80x9cmajority waste solid.xe2x80x9d The majority waste solid is dried and packaged for disposal as secondary waste. Alternatively, the cake can be mixed with ceramic powder to form stabilized pellets after calcination and sintering.
Remaining dissolved metal contaminants present in the secondary salt solution can be removed by reducing the valence or oxidation number of the reducible metallic ions of such metal contaminants to oxidation states of such metallic ions that form slightly soluble or insoluble species of oxides, hydroxides, and similarly-related precipitates. A reducing agent, such as an alkali hyposulfite or an alkali dithionite, is added to reduce the oxidation state of a reducible metallic ion such as chromium from Cr(VI) to Cr(III). Once the oxidation state is reduced, chromium precipitates as insoluble chromium species such as an oxide (Cr2O3) or hydroxide (Cr(OH)3, which can be filtered and disposed of as a xe2x80x9cminority waste solidxe2x80x9d while a xe2x80x9ccleanxe2x80x9d salt solution is collected for eventual recycle to the MSO reactor or further processed.
Generally, the preceding precipitation and filtration steps remove more than about 90% of the metal contaminants that are present in the original or primary spent salt solution. The filtered, clean salt solution obtained in conjunction with the minority waste solid may be further treated depending on whether the salt is to be disposed of or reused. If the salt is destined for re-use, it can be dried using a spray dryer and returned to the MSO reactor. If the salt is to be disposed of, a further clean-up step is necessary. The additional clean-up is accomplished by sending the cleaned solution through, for example, a commercially available ion exchange column (such as Diphonix(trademark)), which yields xe2x80x9csuper-cleanxe2x80x9d salt solutions that contain less than 0.1 ppm of metal contaminants.